Specific protein phosphorylation during stimulation of amylase secretion by beta-agonists or dibutyryl adenosine 3',5'-monophosphate in the rat parotid gland

Neurotransmitter Control of Secretion

It is very well established that the principal control of salivary secretion is derived from autonomic innervation. Transmission of a neural signal to a salivary gland acinar cell occurs chemically via neurotransmitters, the first messengers of a secretory response. Neurotransmitters bind to specific cell surface receptor proteins, an event which activates precise transduction mechanisms which then transfer the neural signal to the inside of the cell. There are two major transduction mechanisms operative in salivary gland acinar cells. One involves the generation of cAMP, the other involves the breakdown of plasma membrane polyphosphoinositides. For both mechanisms, the appropriate stimulated receptor activates a second plasma membrane protein, termed an N (or G) protein. The N protein requires GTP to activate an enzyme (adenylate cyclase or phospholipase C), which then catalyzes the formation of a second messenger (cAMP and inositol trisphosphate/diacylglycerol, respectively). This action provides the intracellular signal for secretory events (protein, fluid, electrolyte secretion) to begin.

Proteinase-activated receptor-2 (PAR-2): regulation of salivary and pancreatic exocrine secretion in vivo in rats and mice

Proteinase-activated receptor-2 (PAR-2) is expressed throughout the gastrointestinal tract including the pancreas, and may be involved in digestive functions. The aim of our study was to evaluate a potential role for PAR-2 in regulating salivary and pancreatic exocrine secretion in vivo. PAR-2-activating peptides (PAR-2-APs), but not selective PAR-1-APs, administered intravenously, increased salivary secretion in the mouse or rat; this effect of the PAR-2-APs was unaffected by atropine, phentolamine, propranolol or indomethacin. Secretion (amylase) by rat parotid gland slices in vitro was also stimulated by PAR-2-APs and trypsin, but not by activation of other PARs. PAR-2-APs, administered to rats in vivo, caused a prompt effect on pancreatic exocrine secretion. PAR-2 mRNA, known to be present in pancreatic tissue, was also detected in parotid tissue. Our results indicate that in addition to a potential role in regulating cardiovascular and respiratory functions, PAR-2 may also play a general role in vivo for the direct regulation of glandular exocrine secretion.

Calcium-calmodulin-stimulated phosphorylation of rat parotid secretion granule proteins

In studies designed to determine the mechanism by which Ca++ and calmodulin stimulate the fusion of parotid secretion granules with plasma membrane vesicles, the hypothesis tested was that Ca++ and calmodulin act by stimulating protein phosphorylation. It was earlier found that Ca++ and calmodulin, but neither alone, stimulated the phosphorylation of four secretion granule proteins with molecular masses of 64, 58, 55 and 31 kDa, and decreased the degree of phosphorylation of a 36-kDa protein. Further studies have shown that in the presence of an optimal concentration of calmodulin (2.4 microM), half-maximal activation of phosphorylation of the four proteins occurred at approx. 8 microM Ca++, and at a maximally effective Ca++ concentration (10(-4) M), half-maximal stimulation occurred at calmodulin concentrations between 0.13 and 1.1 microM for the different proteins. The studies now described also demonstrate that the need for calmodulin for stimulating the phosphorylation, but not the dephosphorylation, is specific; two other Ca(++)-binding proteins, parvalbumin and troponin, could not replace calmodulin in stimulating phosphorylation of the four secretion granule proteins, but either one could substitute for calmodulin in stimulating dephosphorylation of the 36-kDa protein. Additionally, the phosphorylated proteins appear to be located on the granule surface. When secretion granules were subjected to mild treatment with a concentration of trypsin that did not lyse the granules, the 31-, 36-, 55-, 58- and 64-kDa proteins were no longer observed. In the presence of optimal concentrations of Ca++ and calmodulin, a dose-dependent inhibition of the phosphorylation of the various proteins by two calmodulin antagonists, trifluoperazine and calmidazolium, was observed; 50% inhibition of phosphorylation of the different proteins was obtained at approx. 20-40 microM trifluoperazine and at about 2.5-3.0 microM calmidazolium. Inhibition of the dephosphorylation of the 36-kDa protein required greater concentrations of trifluoperazine and calmidazolium; 128 microM and 50 microM, respectively. These results are consistent with the hypothesis that the phosphorylation of one or more of the 31-, 55-, 58- and 64-kDa proteins, but not the dephosphorylation of the 36-kDa protein, may be involved in the action of Ca++ and calmodulin in secretion granule-plasma membrane fusion.

Effects of calcium and calmodulin on the binding of rat parotid secretion granules to the plasma membrane

Since numerous studies suggest that Ca++ and calmodulin may modulate the fusion of secretion granules to the plasma membrane which takes place in exocytosis, we have examined the role of calcium and calmodulin in the binding of isolated parotid secretion granules to plasma membrane vesicles. 125I-labeled inside-out plasma membrane vesicles were incubated with secretion granules, the mixture was layered over 20% sucrose, the gradient was centrifuged, and the amount of 125I in the granule pellet was determined. Addition of Ca++ (20 nM to 10 microM) produced a concentration-dependent increase in the binding of 125I-labeled plasma membrane vesicles to the secretion granules, reaching a maximum value at 10 microM free Ca++; half-maximal binding occurred at 400 nM. Neither right-side-out parotid plasma membrane vesicles nor inside-out pancreatic islet plasma membrane vesicles bound to granules in the presence of 1 microM Ca++. Calmodulin produced a concentration-dependent increase in binding above that of Ca++ alone, and this effect was inhibited by the calmodulin antagonists, trifluoperazine and calmidazolium. Incubation of secretion granules with octadecylrhodamine B (R18)-loaded inside-out plasma membrane vesicles and 2 microM Ca++ caused de-quenching of fluorescence, indicating that the lipids in the granule membrane and the plasma membrane had intermixed. Added calmodulin increased the fluorescence two-fold above that with Ca++ alone. These results suggest that Ca++ and calmodulin may play a role in parotid gland exocytosis by modulating the interaction between the secretion granules and plasma membrane.

Immunohistochemical and immunoblotting identification of protein phosphatase 1 gamma 1 in rat salivary glands

We have analyzed the distribution of the gamma 1 isotype of rat protein phosphatase type 1 catalytic subunit in rat salivary glands. Formaldehyde-fixed paraffin sections were reacted with the PP1 gamma 1 antibody using an immunohistochemical method. Positive staining occurred in striated ducts of parotid gland. However, the staining reaction was less intense in submandibular gland. Proteins were also prepared from rat salivary glands and subjected to SDS-PAGE, followed by Western blotting analysis with the PP1 gamma 1 antibody. The antibody interacted with protein corresponding to an estimated molecular mass of 36 kDa present in the parotid gland. The staining reaction was considerably weaker with the proteins from submandibular gland.

Effect of vanadate on amylase secretion and protein tyrosine phosphatase activity in the rat parotid gland

Treatment of rat parotid acinar cells with sodium orthovanadate (an inhibitor of protein tyrosine phosphatase) caused a dose-dependent inhibition of phosphatase activity as measured by the hydrolysis of para nitrophenylphosphate (pNPP). Inclusion of 50 microM sodium orthovanadate in in vitro gland cultures prevented the amylase secretion from both untreated control and isoproterenol-stimulated parotid acinar cells. Four different tyrosine-phosphorylated proteins with M(r) 40, 70 and 95 kDa, respectively, were identified in secretory granule preparations from rat parotid glands by immunoblot using a monospecific antibody for phosphotyrosine. An increase in the phosphorylation levels of these phosphoproteins was noted in the presence of 50 microM sodium orthovanadate, suggesting that a protein tyrosine phosphatase (PTPase) is involved in parotid gland protein dephosphorylation reactions. Using antibody to Syp (a PTPase belonging to class 1D), a major fraction of subcellular activity was found to be associated with secretory granule membranes. These results suggest the possible involvement of a PTPase (Syp) in parotid gland secretory mechanisms.

Characterization of human and rat immortalized clones parotid acinar cells with respect to specific proteins and their mRNAs, and receptor-linked adenylate cyclase

This study reports the isolation and characterization of a rat nontumorigenic parotid acinar cell clone (2RSG), a human nontumorigenic parotid acinar cell clone (2HPC8), and a human tumorigenic acinar clone (2HP1G). The levels of alpha-amylase mRNAs detected when using alpha-amylase cDNA of 1176 and 702 bp for hybridization were higher in 2RSG and 2HPC8 cells than their respective whole parotid glands. The level of these mRNAs decreased in 2HP1G cells. In contrast to alpha-amylase mRNAs levels, the alpha-amylase activity in cultured acinar cells was extremely low in comparison to whole glands, irrespective of species or cell status. The levels of proline-rich protein (PRP) mRNA and parotid secretory protein (PSP) mRNA detected when using PRP cDNA of 600 bp and PSP cDNA of 805 bp for hybridization were higher in 2RSG cells than those in rat parotid glands; the reverse was observed in 2HPC8 cells and human parotid glands. The levels of PRP mRNA and PSP mRNA in 2HPC8 and 2PH1G acinar cells were similar. The level of mRNA was not detectable in murine neuroblastoma cells (NBP2) using the same alpha-amylase cDNA, PRP cDNA and PSP cDNA for hybridization. The PSP level in rat parotid gland was lower than that found in 2RSG cells; the reverse was observed in 2HPC8 cells and human parotid glands. The level of PSP in 2HP1G cells was higher than that found in 2HPC8 cells. Isoproterenol increased the cAMP level in 2RSG, 2HPC8, and 2HP1G clones, being most effective in 2RSG cells, and least effective in 2HPG cells. Prostaglandin E1 (PGE1) also increased cAMP level, being most effective in 2HPC8 cells and ineffective in 2HP1G cells, suggesting that the PGE1 receptor-linked adenylate cyclase becomes inactive upon transformation. These results suggest that the three clonal acinar cells from rat and human parotid glands reported here can be useful in comparative studies on regulation of growth, differentiation, and transformation.

The dephosphorylation of 22-kDa phosphoprotein by type 2B protein phosphatase in rat parotid acinar cells

In saponin-permeabilized rat parotid acinar cells, cyclic AMP and 3-isobutyl-1-methylxanthine stimulated the phosphorylation of three particulate proteins with molecular masses of 34, 26 and 22 kDa. The particulate fractions containing 22-kDa phosphoprotein were isolated from the cells labelled with [gamma-32P]ATP and used to study the dephosphorylation of the 22-kDa phosphoprotein. When the labelled fractions were incubated at 30 degrees C in the presence of 0.3 mM CaCl2 and 10 micrograms calmodulin, dephosphorylation of the 22-kDa phosphoprotein was evoked. Further addition of the type 2B phosphatase (Ca2+/calmodulin-stimulated protein phosphatase purified from bovine brain) resulted in a remarkable dephosphorylation of the 22-kDa phosphoprotein. Western immunoblotting showed that type 2B protein phosphatase exists in rat parotid acinar cells. These results suggest that type 2B protein phosphatase in those cells is involved in the dephosphorylation of the 22-kDa phosphoprotein.

Purification and characterization of multiple S6 phosphatases from the rat parotid gland

S6 phosphatase activities, which dephosphorylate the phosphorylated S6 synthetic peptide, RRLSSLRASTSKSESSQK, were purified to near homogeneity from the membrane and cytosolic fractions of the rat parotid gland. Multiple S6 phosphatases were fractionated on Mono Q and gel filtration columns. In the cytosolic fraction, at least three forms of S6 phosphatase, termed peaks I, II, and III, were differentially resolved. The three forms had different sizes and protein compositions. The peak I enzyme, which had an approximately Mr of 68 kDa on gel filtration, appears to represent a dimeric form of the 39 kDa protein. This S6 phosphatase showed the high activity in the presence of EGTA and was completely inhibited by nanomolar concentrations of either okadaic acid or inhibitor 2. The peak II S6 phosphatase enzyme, with an Mr of 35 kDa, was activated by Mn2+. This form could be a proteolytic product of the catalytic subunit of type 1 phosphatase, due to its sensitivities to okadaic acid and inhibitor 2. The peak III enzyme, with an Mr of 55 kDa, is a Mn(2+)-dependent S6 phosphatase. This S6 phosphatase can be classified as a type 1 phosphatase, due to its sensitivity to okadaic acid, since the IC50 of okadaic acid is 4 nM. However, the molecular mass of this S6 phosphatase differs from that of the type 1 catalytic subunit (37 kDa) and showed less sensitivity to inhibitor 2. On the other hand, the membrane fraction contained one form of the S6 phosphatases, termed peak V (Mr 34 and 28 kDa), which could be classified as a type 1 phosphatase. This S6 phosphatase activity was greatly stimulated by Mn2+.

The role of phosphotyrosine signaling pathway in a parotid gland proliferation and function

Tyrosine phosphorylation and the intracellular signaling processes associated with it have been the focus of intense study due to its importance in the regulation of biological processes as diverse as cell proliferation and cell differentiation. While much of what we now understand has been derived from the study of cell lines and tumor cells, the salivary glands provide a model to examine the effects of tyrosine kinases and tyrosine phosphatases in a normal differentiated tissue. This review will focus, therefore, on the role tyrosine kinases and phosphatases play in inducing the transition from stasis to active proliferation and their potential role in mediating secretory function of the salivary glands.

Possible involvement of adenylylation in the modification of a 26 kDa protein in rat parotid acinar cells

1. Adenylylation, a posttranslational modification of proteins, was investigated in saponin-permeabilized acinar cells of the rat parotid gland. 2. When cells were incubated with [2,8-3H]ATP, several proteins, including a 26 kDa protein in the particulate fraction, were labeled. 3. Upon incubation of cells with [alpha-32P]ATP in the presence of cAMP and 3-isobutyl-1-methylxanthine, 32P-labeling of the 26 kDa protein was observed. 4. After treatment with snake venom phosphodiesterase, [32P]AMP was released from the 26 kDa protein. Such release was not observed when cells were labeled with [gamma-32P]ATP. 5. The 32P-labeling pattern of proteins with [alpha-32P]ATP was clearly different from that with [adenylate-32P]NAD+. 6. The results suggest that the 26 kDa protein is one of the adenylylation substrates in rat parotid acinar cells.

Evidence for the involvement of protein phosphorylation in cyclic AMP-mediated amylase exocytosis from parotid acinar cells

We evaluated the role of protein phosphorylation in cAMP-mediated amylase exocytosis from parotid acinar cells by using H89, a new protein kinase A (PKA) inhibitor, which is more lipophilic and 25 times more potent than H8. In our previous studies, H8 markedly inhibited protein phosphorylation without decreasing amylase release [Takuma, T. (1988) Biochem. J. 256, 867-871]. These findings were completely reproduced even in the small acini that were prepared by trypsin treatment before collagenase digestion. In the present study, however, H89 strongly inhibited both amylase release and protein phosphorylation in a dose-dependent manner. The inhibitory effect was specific for PKA at least up to 33 microM, since 33 microM H89 did not block amylase release stimulated by PMA. H85, a closely related compound of H89 without inhibitory effect on PKA, did not prevent amylase release or protein phosphorylation at least up to 33 microM. These results suggest that protein phosphorylation by PKA is involved in cAMP-mediated amylase exocytosis. The inhibition of protein phosphorylation by H8 might be insufficient or inadequate for blocking of amylase release.

Signaling mechanisms that regulate saliva formation

The precipitating event in the formation of saliva is the binding of neurotransmitter molecules to cell surface receptor proteins. The principal neurotransmitters involved are acetylcholine and norepinephrine that bind, respectively, to muscarinic-cholinergic, and alpha- and beta-adrenergic receptors. The transduction of the extracellular signal requires an integral membrane protein capable of binding GTP, a G protein, that specifically interacts with the receptor. The components of G protein transduction systems are fairly well studied, but the pathways by which signals are routed are just being recognized. Delineation of such routing pathways is essential to understanding the regulation of saliva formation.

Rat submandibular and parotid protein phosphorylation and exocytosis: effect of site-selective cAMP analogs

A series of cAMP analogs that have different specificities for the two different binding sites on the regulatory subunit of type I and type II cAMP-dependent protein kinase (PKA) were used to determine whether selective activation of type I or type II PKA could link either or both isozyme forms of PKA with exocytosis and specific protein phosphorylation in salivary gland cells. Using dispersed rat submandibular or parotid cells, selective activation of either type I or type II resulted in a synergistic response for both rat submandibular mucin and parotid amylase secretion and the phosphorylation of a 26-kDa integral membrane phosphoprotein. These data suggest that the activation of either isozyme of PKA can elicit cellular exocytosis and specific protein phosphorylation in both of these cell types.

Phosphorylation of elongation factor 2 during Ca(2+)-mediated secretion from rat parotid acini

In this paper we report the rapid phosphorylation of a cytosolic 100 kDa protein during stimulation of secretion from dispersed aggregates of parotid acinar cells with Ca(2+)-mobilizing secretagogues (carbachol, Substance P, ATP and the Ca2+ ionophore A23187). Phosphorylation was inhibited by removal of extracellular Ca2+ but was not observed during stimulation with phorbol esters, suggesting that this protein is not a substrate for protein kinase C. Two-dimensional PAGE and immunoprecipitation with a specific antiserum indicated that this protein is elongation factor 2, whose Ca2+ calmodulin-dependent phosphorylation has been shown to inhibit protein synthesis [Nairn & Palfrey (1987) J. Biol. Chem. 262, 17299-17303]. These results suggest that phosphorylation of elongation factor 2 is the molecular mechanism for the inhibition of protein synthesis which has been previously observed in rat parotid cells during stimulation with Ca(2+)-mobilizing secretagogues.

Purification and partial characterization of analogous 26-kDa rat submandibular and parotid gland integral membrane phosphoproteins that may have a role in exocytosis

Rat submandibular and parotid gland exocytosis is primarily controlled by beta-adrenergic receptor stimulation. Although its precise role in the regulation of salivary gland exocytosis is not fully understood, protein phosphorylation, mediated by the activation of cAMP-dependent protein kinase, may be directly involved. Previous studies suggest that analogous 26-kDa integral membrane phosphoproteins may play a direct role in regulating exocytosis. Studies were here undertaken to purify and partially characterize both phosphoproteins. After endogenous phosphorylation with 32P, subcellular fraction and solubilization of the microsomal fraction in n-octyl beta-glucopyranoside, the 26-kDa integral membrane phosphoproteins were purified by high performance liquid chromatography (HPLC), followed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and electroelution of the proteins. Amino acid analysis indicated a significant number of serine amino acids: N-terminal sequence data demonstrated a high level of homology; and trypsin digestion followed by reversed-phase HPLC indicated the possibility of multiple phosphorylation sites.

Isoproterenol-stimulated labelling of particulate proteins by using [adenylate-32P]NAD+ independent on a cAMP-dependent protein kinase in parotid acinar cells

When saponin-permeabilized rat parotid acinar cells were incubated with [adenylate-32P]NAD+, labelling of proteins (33, 27 and 23 kDa) in particulate fractions of the cells was stimulated by isoproterenol. The effect of isoproterenol was completely blocked by a beta-antagonist. Both forskolin or cAMP mimicked the effect of isoproterenol on the labelling. However, an inhibitor of cAMPdPK failed to induce complete inhibition of the effects of isoproterenol, forskolin and cAMP. When the labelled proteins were treated with snake venom phosphodiesterase, neither [32P]5'-AMP nor [32P]phosphoribosyladenosine was released. These results suggest that covalent modification of proteins with NAD+, which is distinct from ADP-ribosylation and cAMPdPK-dependent phosphorylation, is coupled to beta-receptor-cAMP signalling system in rat parotid acinar cells.

cAMP-dependent phosphorylation of RER proteins from rat liver: relationship with GTP-dependent membrane fusion

Incubation of stripped rough microsomes (SRM) with the catalytic subunit of protein kinase A (PKA) permitted specific phosphorylation of seven proteins having relative molecular mass values of 55, 35, 23, 22.5, 22, 18.5 and 16.5 kDa (P55, P35 etc.). By two dimensional gel analysis, we compared these phosphoproteins with low-molecular-weight GTP-binding proteins and revealed that P23 and P22.5 co-migrated with known GTP-binding proteins. Next we examined the effect of cAMP-dependent phosphorylation on a GTP-dependent membrane function, membrane fusion. Quantitative analysis indicated no difference in the amount of membrane fusion obtained whether SRM were incubated in the absence or in the presence of PKA. Thus several rough microsomal proteins underwent cAMP-dependent phosphorylation and this post-translational modification did not affect GTP-dependent membrane fusion in a cell free system.

Okadaic acid inhibits amylase exocytosis from parotid acini stimulated by cyclic AMP

To evaluate the role of protein phosphorylation in amylase exocytosis, we studied the effects of okadaic acid, a potent inhibitor of protein phosphatase types 1 and 2A, on amylase release and protein phosphorylation in rat parotid acini. Although okadaic acid by itself weakly stimulated amylase release, it did not potentiate amylase release stimulated by half-maximum doses of isoproterenol or cAMP, and markedly inhibited their maximum effects. Okadaic acid dose-dependently increased cAMP-independent phosphorylation of some proteins and enhanced cAMP-dependent phosphorylation of 21- and 26-kDa proteins. These results indicate that increase in protein phosphorylation does not necessarily enhance the exocytosis of amylase from parotid acini.

Localization of catalytic and regulatory subunits of cyclic AMP-dependent protein kinases in mitochondria from various rat tissues

Observation and quantification of the catalytic subunit C of cyclic AMP-dependent protein kinases by immuno-gold electron microscopy suggested a high concentration of cyclic AMP-dependent protein kinases in mitochondria from liver, kidney, heart and skeletal muscle, pancreas, parotid gland and brain cells. The position of gold particles pointed to a localization in the inner membrane/matrix space. A similar distribution was obtained by immunolocalization of the cyclic AMP-dependent protein kinase regulatory subunits RI and RII in liver, pancreas and heart cells. The results indicated the presence of both the type I and the type II cyclic AMP-dependent protein kinases in mitochondria of hepatocytes, and the preferential occurrence of the type I protein kinase in mitochondria from exocrine pancreas and heart muscle. The immunocytochemical results were confirmed by immunochemical determination of cyclic AMP-dependent protein kinase subunits in fractionated tissues. Determinations by e.l.i.s.a. of the C-subunit in parotid gland cell fractions indicated about a 4-fold higher concentration of C-subunit in the mitochondria than in a crude 1200 g supernatant. Immunoblot analysis of subfractions from liver mitochondria supported the localization in situ of cyclic AMP-dependent protein kinases in the inner membrane/matrix space and suggested that the type I enzyme is anchored by its regulatory subunit to the inner membrane. In accordance with the immunoblot data, the specific activity of cyclic AMP-dependent protein kinase measured in the matrix fraction was about twice that measured in whole mitochondria. These findings indicate the importance of cyclic AMP-dependent protein kinases in the regulation of mitochondrial functions.

Residual secretion of amylase induced by isoproterenol from rat parotid gland slices

1. During pretreatment with isoproterenol (ISP) for 2 min the accumulation of cyclic AMP increased in a dose-dependent manner but amylase release did not increase even in the highest dose of ISP used. 2. After a brief pretreatment with ISP, the following 10-min incubations with fresh medium without ISP caused increase in amylase secretion (residual secretion). However, cyclic AMP accumulation returned to the non-treated level during the residual secretion of amylase. 3. Both residual amylase release and cyclic AMP accumulation after pretreatment with ISP were enhanced in the presence of isobutyl-methylxanthine. Residual amylase release was not affected in the absence of extracellular calcium ion. 4. We suggest that there may be another pathway than cyclic AMP to cause residual amylase secretion induced by brief pretreatment with ISP in rat parotid acinar cells.

Co-ordinated changes in the cyclic AMP signalling system and the phosphorylation of two nuclear proteins of Mr 130,000 and 110,000 during proliferative stimulation of the rat parotid gland by isoprenaline. Possible identity of the two proteins with pp135 and nucleolin

Parotid glands were stimulated to growth by repeated injection of the beta-agonist isoprenaline into rats. Incubation of intact parotid-gland lobules with [32P]Pi and subsequent analysis of nuclear proteins revealed in the stimulated glands an increased 32P incorporation into two acid-soluble non-histone proteins with apparent Mr values of 110,000 and 130,000 (p110 and p130). After a single injection of isoprenaline, leading to a biphasic increase in DNA synthesis (maximum at 24 h), the same two proteins showed a transiently increased 32P incorporation at 17 h after injection. At this time point at the onset of DNA synthesis the total activity of soluble cyclic AMP-dependent protein kinase decreased. No change in p110/p130 phosphorylation was observed at 0.3 h after stimulation, a time of maximal stimulation of secretion. Administration of the beta-antagonist propranolol 8 h after the injection of isoprenaline suppressed the increase in DNA synthesis, the preceding changes in the concentration of cyclic AMP and in the activity of cyclic AMP-dependent protein kinase, as well as the increased phosphorylation of p110 and p130. Cross-reactivity of p110 and p130 with specific antisera against two nucleolar phosphoproteins of similar molecular mass (nucleolin and pp135), as well as their localization in a nucleolar cell fraction, indicated a possible identity of p110 and p130 with these two proteins. Our results suggest that nucleolin and pp135 are nuclear target proteins of cyclic AMP in the cyclic AMP-influenced regulation of the transition of cells from the G1 to the S phase.

Modulation of pp60c-src tyrosine kinase activity during secretion in stimulated bovine adrenal chromaffin cells

High levels of the proto-oncogene product, pp60c-src, have been found in developing and adult neural tissues as well as in certain fully mature cells of the hematopoietic lineage, e.g., platelets and myelomonocytes. Adrenal medullary chromaffin cells exhibit characteristics of both types of cells, i.e., they are derived from the neural crest and carry out exocytosis in response to specific stimuli. Earlier studies have shown that pp60c-src localizes not only to the plasma membrane of chromaffin cells but also to the membranes of chromaffin granules, the secretory vesicles of these cells that store catecholamines and other secretory products. To investigate the possible involvement of pp60c-src in exocytosis, cultured bovine chromaffin cells were analyzed for changes in c-src tyrosine kinase activity in response to stimulation by several secretagogues. Results of in-vitro immune complex kinase assays showed that pp60c-src, derived from cells that had been stimulated for various lengths of time, exhibited decreased auto- and transphosphorylating activities as compared to pp60c-src immunoprecipitated from control cells. The greatest reduction in activity was observed 10 min post-stimulation, while normal levels were regained 2-6 hr after secretagogue treatment. Western immunoblot analysis of the immunoprecipitated pp60c-src revealed that approximately 50% less c-src protein was present in immune complexes prepared 10 min after stimulation as compared to those prepared from mock-stimulated controls, resulting in a specific autophosphorylating activity that was 42-47% of control and little or no reduction in the transphosphorylating specific activity. In experiments in which the rate of secretion of [3H]-norepinephrine from cells preloaded with this compound was compared to the rate of modulation of pp60c-src activity, 50% of the maximal reduction in pp60c-src activity occurred within 2-4 min while 50% maximal release of [3H]-norepinephrine occurred within 1-3 min. Taken together, these results suggest that pp60c-src may play some role (direct or indirect) in the exocytotic process.

Evidence against direct involvement of cyclic AMP-dependent protein phosphorylation in the exocytosis of amylase

To examine whether or not the activation of cyclic AMP-dependent protein kinase is coupled to the exocytosis of amylase from rat parotid cells, the effect of protein kinase inhibitors on amylase release and protein phosphorylation was studied. A membrane-permeable inhibitor of cyclic AMP-dependent protein kinase, N-[2-(methylamino)ethyl]-5-isoquinolinesulphonamide (H-8), and peptide fragments of the heat-stable protein kinase inhibitor [PKI-(5-24)-peptide and PKI-(14-24)-amide] strongly inhibited cyclic AMP-dependent protein kinase activity in the cell homogenate. However, H-8 had no inhibitory effect on amylase release from either intact or saponin-permeabilized parotid cells stimulated by isoproterenol or cyclic AMP. Moreover, PKI-(5-24)-peptide and PKI-(14-24)-amide did not inhibit cyclic AMP-evoked amylase release from saponin-permeabilized cells, whereas cyclic AMP-dependent phosphorylations of 21 and 26 kDa proteins in intact or permeabilized cells were markedly inhibited by these inhibitors. These results suggest that cyclic AMP-dependent protein phosphorylation is not directly involved in the exocytosis of amylase regulated by cyclic AMP.

cAMP-dependent protein phosphorylation of membrane proteins in the parotid gland, platelets and liver. Comparison of a 22-kDa phosphoprotein from rat parotid microsomes (protein III) with phosphoproteins of similar molecular size from platelet and liver membranes

Stimulation of secretion in exocrine secretory glands leads to the phosphorylation of a 22-kDa membrane protein (protein III) whose function is still unknown [Jahn et al. (1980) Eur. J. Biochem. 112, 345-352; Jahn & Söling (1980) Proc. Natl Acad. Sci. USA 78, 6903-6906]. This report describes the comparison of this protein with phosphorylated membrane proteins of similar molecular mass in platelets and liver. Incubation of platelets with agents which raise the intracellular cAMP concentration results in the phosphorylation of a 22-kDa protein which is also phosphorylated in membrane preparations by endogenous kinases or by exogenous cAMP-dependent protein kinase. It is shown that this protein is distinct from protein III although both proteins have the same molecular mass and are substrates of cAMP-dependent protein kinase. In contrast to platelets, protein III could be demonstrated in liver microsomes. This indicates that the function of protein III is not exclusively linked to the stimulus-secretion coupling in exocrine cells.

Purification and characterization of a 22-kDa microsomal protein from rat parotid gland which is phosphorylated following stimulation by agonists involving cAMP as second messenger

Stimulation of secretion in exocrine glands by agonists involving cAMP as second messenger leads to the phosphorylation of the ribosomal protein S6 (protein I) and two other particulate proteins with apparent molecular masses of 24 kDa (protein II) and 22 kDa (protein III) [Jahn, R., Unger, C. & Söling, H. D. (1980) Eur. J. Biochem. 112, 345-352]. This report describes the purification and characterization of protein III. Solubilization studies indicate that protein III is an intrinsic membrane protein. It could be extracted from the endoplasmic reticulum membrane only with Triton X-100, SDS or concentrated formic or acetic acid. The purification of this protein involved extraction of the microsomes with Triton X-100, removal of the detergent by acetone precipitation, extraction of water-soluble proteins, lipids and lipoproteins, and preparative SDS polyacrylamide gel electrophoresis. The protein has a basic pI (greater than 8.7). For determination of the amino acid composition of protein III and for sequencing of its amino-terminal portion, the protein was electroeluted out off the gel, the detergent removed and the protein finally purified by reversed-phase HPLC. Protein III could be phosphorylated in vitro by the catalytic subunit of the cAMP-dependent protein kinase to a degree of approximately 0.14 mol phosphate/mol protein. The only phosphopeptide obtained after in vitro phosphorylation and subsequent tryptic or chymotryptic digestion was identical with the phosphopeptide obtained after stimulation of intact rat parotid gland lobules with isoproterenol. The sequence of this peptide was Lys-Leu-Ser(P)-Glu-Ala-Asp-Asn-Arg. It was confirmed by an analysis of the synthetic peptide following in vitro phosphorylation with cAMP-dependent protein kinase. The first 41 N-terminal residues of protein III were sequenced. So far no sequence homology with other known peptides or proteins could be found.

Selective regulation of the amount of catalytic subunit of cyclic AMP-dependent protein kinases during isoprenaline-induced growth of the rat parotid gland

Stimulation of growth of the rat parotid gland by repeated injection of the beta-agonist isoprenaline led to a significant decrease in the activity of cyclic AMP-dependent protein kinases. Immunochemical quantification of the catalytic (C) and regulatory (RI and RII) subunits of the cyclic AMP-dependent protein kinases type I and type II revealed a loss of 65% of the immunochemically measurable amount of catalytic subunit C. The amount of the regulatory subunits, however, remained constant. The observed decrease in C-subunit was not due to a translocation of the molecule to cellular membranes or to an inhibiting effect of the heat-stable inhibitor of cyclic AMP-dependent protein kinases. A selective decrease in only the C-subunit was also observed after a brief exposure to isoprenaline leading to the stimulation of DNA synthesis. Under these conditions, the decrease was observed at the onset of DNA synthesis (17 h after injection), but not at the the time of an earlier small cyclic AMP peak (13 h after injection) or at the time of maximal DNA synthesis (24 h after injection). The results indicate that the amount of the catalytic subunit of cyclic AMP-dependent protein kinases can be regulated independently from that of the regulatory subunits. The time-limited occurrence of the specific change in the amount of the C-subunit suggests that such a regulation is of physiological significance and that it may participate in cyclic AMP-mediated events involved in the control of cellular proliferation.

Protein phosphatases of the guinea-pig parotid gland

The nature of protein phosphatases of the guinea-pig parotid gland was investigated. The protein phosphatases were characterized by (a) the use of five different 32P-labelled substrate proteins (phosphorylase a, histone H2B, casein, and the alpha and beta subunits of phosphorylase kinase), (b) their behaviour during ion-exchange chromatography, (c) their relative molecular mass distribution during gel filtration, (d) their sensitivity towards inhibition by inhibitor 2, (e) their ability to be stimulated by protamine and (f) by their behaviour during freezing and thawing in the presence of 2-mercaptoethanol. The following results were obtained. 1. The 'cytosol' (100,000 X g supernatant) contains protein phosphatases of the types 1, 2A and 2B. 2. On the basis of inhibition with inhibitor 2 (1.2 micrograms/ml) the 'cytosolic' phosphorylase phosphatase activity consists to about 40% of protein phosphatase 1 and to about 60% of protein phosphatase 2A. 3. In the cytosol about 80-90% of the protein phosphatases 1 and 2A exist in an inactive state. 4. A 5-10-fold activation can be achieved by ethanol precipitation, which results in the generation of a mixture of forms of low apparent molecular mass of about 30 kDa. 5. Microsome-associated phosphorylase phosphatase activities can be extracted in a highly active state by detergent (1% Triton X-100) or by 0.8 M NaCl. 6. Activity measurements in the presence of inhibitor 2 (1.2 micrograms/ml) indicate that the microsomal activities consist to about 75% of protein phosphatase 1 and to about 25% of protein phosphatase 2A. Activities corresponding to protein phosphatases 2B and 2C could not be detected. 7. The 'microsomal' protein phosphatase activities exhibit lower apparent molecular masses (70 kDa and 30 kDa) than the 'cytosolic' protein phosphatases (about 260 kDa). 8. After ethanol treatment of the microsomal protein phosphatases only activities with apparent molecular masses of about 30 kDa can be detected. These share several similarities with the ethanol-treated cytosolic protein phosphatases. 9. Both cytosolic and microsomal protein phosphatases display activity towards histone H2B and casein.

cAMP-mediated protein phosphorylation of microsomal membranes increases mannosylphosphodolichol synthase activity

We have investigated the possible role of a cAMP-mediated protein-phosphorylation event(s) as the key regulatory mechanism in beta-adrenoreceptor-stimulated activation of mannosylphosphodolichol (Man-P-Dol) synthase (GDP-mannose:dolichyl-phosphate O-beta-D-mannosyltransferase, EC 2.4.1.83) in rat parotid acinar cells. Microsomal membranes isolated from these cells pretreated with 10 microM isoproterenol for 60 min showed approximately 40-80% enhanced Man-P-Dol synthase activity compared to the untreated controls. This change in enzyme activity was not associated with a significant alteration in apparent Km for GDP-mannose, but the Vmax was enhanced 2-fold. When microsomal membranes isolated from control cells were phosphorylated in vitro by a cAMP-dependent protein kinase, an increase in Man-P-Dol synthase activity, similar to that with membranes from isoproterenol-treated cells, was observed (i.e., a moderate change in Km for GDP-mannose but a 2-fold higher Vmax). Furthermore, treatment of in vitro phosphorylated microsomal membranes by alkaline phosphatase led to a substantial reduction in Man-P-Dol synthase activity. Increased Man-P-Dol synthesis (approximately 30-40%) was also observed in bovine brain and hen oviduct microsomal membranes after in vitro protein phosphorylation. In aggregate, these results strongly suggest that agents that increase cAMP in cells may modulate protein N-glycosylation in those cells by activating this key glycosyltransferase of the dolichol cascade by a cAMP-dependent protein kinase-mediated protein phosphorylation/dephosphorylation cycle.

Neurotransmitter control of secretion

It is very well established that the principal control of salivary secretion is derived from autonomic innervation. Transmission of a neural signal to a salivary gland acinar cell occurs chemically via neurotransmitters, the first messengers of a secretory response. Neurotransmitters bind to specific cell surface receptor proteins, an event which activates precise transduction mechanisms which then transfer the neural signal to the inside of the cell. There are two major transduction mechanisms operative in salivary gland acinar cells. One involves the generation of cAMP, the other involves the breakdown of plasma membrane polyphosphoinositides. For both mechanisms, the appropriate stimulated receptor activates a second plasma membrane protein, termed an N (or G) protein. The N protein requires GTP to activate an enzyme (adenylate cyclase or phospholipase C), which then catalyzes the formation of a second messenger (cAMP and inositol trisphosphate/diacylglycerol, respectively). This action provides the intracellular signal for secretory events (protein, fluid, electrolyte secretion) to begin.

Phosphorylation and dephosphorylation of chromaffin cell proteins in response to stimulation

Stimulation of bovine chromaffin cell in culture changed (increased or decreased) the phosphorylation state of several proteins as examined by 32P incorporation. Enhanced phosphorylation of 22 protein bands as well as increased dephosphorylation of a 20.4 kilodaltons protein band was observed when extracts of cultured chromaffin cells stimulated by either acetylcholine or high K+ were subjected to mono-dimensional gel electrophoresis. For several protein bands, the degree of phosphorylation was larger in cells stimulated by acetylcholine than in those challenged by a depolarizing concentration of K+. The most affected phosphoproteins have apparent molecular weights of 14,800, 29,000, 33,000, 57,000 (tubulin subunit), 63,000 (tyrosine hydroxylase subunit) and 94,000. The presence of a low extracellular calcium concentration (0.5 mM Ca2+ plus 15 mM Mg2+) in the incubation medium inhibited (38-100%) the acetylcholine-evoked increases in protein phosphorylation observed previously for 18 protein bands. Trifluoperazine at the concentration required for 50% inhibition of acetylcholine-induced catecholamine release decreases (33-100%) the stimulation-induced phosphorylation in all polypeptides, with the exception of the 14.8 kilodaltons and the dephosphorylated 20.4 kilodaltons components which were not affected. Two-dimensional gel electrophoresis analysis revealed that exposure of chromaffin cells to acetylcholine produced two types of effect on protein phosphorylation: activation of protein kinase activities affecting about 30 polypeptides; activation of protein phosphatase activities resulting in the dephosphorylation of about 40 polypeptides, most of them appearing as minor phosphoproteins, with the exception of the alpha-subunit of pyruvate dehydrogenase and the 20.4 kilodaltons polypeptide. On the basis of their molecular properties (molecular weight and pI) and their abundance in chromaffin cells, the 80 kilodaltons phosphoprotein which focused at pI 4.8 and the 117.5 kilodaltons phosphoprotein which focused at pI 5.0 were identified as chromogranins A and B, respectively. The relationship between acetylcholine-induced protein phosphorylation (or dephosphorylation) and catecholamine secretion was also investigated. The time course of protein phosphorylation (or dephosphorylation) paralleled or preceded [3H]noradrenaline release for 16 phosphoproteins.(ABSTRACT TRUNCATED AT 400 WORDS)

Phosphorylation of salivary proteins by salivary gland protein kinase

Human saliva contains a number of phosphorylated acidic proline-rich proteins (APRP). Monkey parotid saliva contains a similar protein with the same phosphorylated sequences as the human proteins. A crude protein kinase was prepared from Macaca fascicularis parotid glands which phosphorylated human APRP. The enzyme was activated by Mg2+, it had a pH optimum between pH 7.0 and 7.5, the Km for ATP was 78 mumol/L, and for APRP it was 85 mumol/L. Phosphorylation of APRP was independent of cAMP and calmodulin. Phosphate was incorporated as phosphoserine, and the kinase phosphorylated the same residues in dephosphorylated APRP which are phosphorylated in the secreted protein. In addition, the enzyme preparation also phosphorylated dephosphorylated and native APRP in a region which is not phosphorylated in the secreted protein. There was no difference in the rate of phosphorylation of APRPs and their tryptic peptides. The kinase also phosphorylated other dephosphorylated salivary phosphoproteins. An enzyme was demonstrated in the human salivary gland which gave the same pattern of phosphorylation of APRP as did the simian kinase. More than one kinase may be necessary for the observed phosphorylation.

Cyclic-AMP-dependent protein phosphorylation in the soluble fraction of parotid glands from rats with drug-induced hypothyroidism

Cyclic-AMP-dependent protein kinase from the soluble fraction of parotid glands of hypothyroid rats was partially purified. Its isozyme distribution and kinetic properties were similar to those of euthyroid rats. Electrophoresis of 100 microliters portions at 20 mA per slab revealed that an endogenous protein (mol. wt 68,000) was specifically phosphorylated in hypothyroidism; this protein was not found in euthyroid rats. In the presence of cyclic AMP, there was stimulated phosphorylation of euthyroid-soluble proteins with molecular weights of 115,000, 98,000, 57,000, 50,000, 44,000, 33,000 and 19,000, and of proteins from hypothyroid rats with weights of 115,000, 98,000, 60,000, 50,000, 33,000 and 19,000. Tolbutamide reduced incorporation of 32Pi into soluble proteins from both groups. However, cyclic AMP still induced phosphorylation in euthyroid preparations in the presence of tolbutamide, but its effect was markedly reduced in the hypothyroid state. These differences in endogenous protein phosphorylation may have different effects on amylase release induced by beta-adrenergic stimulation.

Neuroendocrine control of secretion in pancreatic and parotid gland acini and the role of Na+,K+-ATPase activity

The results of our investigations into the localization of Na+,K+-pump activity in pancreatic and parotid acinar cells and the effects of hormones and neurotransmitters on pump turnover can be integrated with data on other aspects of stimulus-response coupling to construct models of the neurohumoral control of protein, fluid, and electrolyte secretion (Fig. 23). In both tissues, Ca2+ and cyclic AMP serve as intracellular messengers. In pancreatic acinar cells, the Ca2+-dependent pathway activated by the occupation of CCK or cholinergic receptors provides the primary stimulus for digestive enzyme secretion. Cyclic AMP plays a comparatively minor role; VIP and secretin are much less effective stimulators of protein secretion. Conversely, cyclic AMP levels in parotid acinar cells, which are modulated primarily through occupation of beta-adrenergic receptors, are a major determinant of enzyme secretion. Activation of the Ca2+-dependent pathway by cholinergic or alpha-adrenergic agonists or substance P is less important. The presence of dual control processes in each gland suggests that the observed differences in effectiveness of cyclic AMP- versus Ca2+-dependent secretagogues may reflect not different mechanisms, but rather a shift in the relative emphasis placed on each pathway. This emphasis could conceivably result from subtle variations in the interaction between cellular protein kinases and phosphatases and their phosphoprotein substrates. Electrolyte secretion, on the other hand, appears to involve both discrete and common entities. In pancreatic acinar cells from rodent species, cholinergic or CCK receptor occupancy elicits a Ca2+-dependent increase in the open-state probability of nonselective cation channels in the basolateral plasma membrane. The resultant influx of Na+ and efflux of K+ is most probably the factor which activates Na+, K+-pumps. Based on electron probe studies of the effects of cholinergic agonists on acinar cell Na+ and K+ contents discussed earlier, a transient reduction in the intracellular K+/Na+ ratio of up to 4-fold may occur. A shift of this magnitude in the cytoplasmic microenvironment of the Na+, K+-pump clearly would have a stimulatory influence (see discussion by Jorgensen, 1980). In addition, Ca2+ itself may have direct effects on Na+,K+-pump activity. Calcium at levels much above 1 microM progressively inhibits Na+,K+-ATPase activity (Tobin et al., 1973; Yingst and Polasek, 1985). In unstimulated guinea pig pancreatic acinar cells, Ca2+i measured by quin-2 fluorescence was 161 +/- 13 nM (Hootman et al., 1985a) which increased to a maximal concentration of 803 +/- 122 nM following CCh stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

The rate-determining step in cAMP-mediated exocytosis in the rat parotid and submandibular glands appears to involve analogous 26-kDa integral membrane phosphoproteins

The possible direct involvement of protein phosphorylation in the regulation of exocytosis during beta-adrenergic receptor stimulation in rat parotid and submandibular salivary glands was investigated in vitro using dispersed cells. The dispersed cells were labeled with [32P]orthophosphate for 40 min prior to experimental manipulation. Subcellular fractions were isolated, the proteins were separated using sodium dodecylsulfate/polyacrylamide gel electrophoresis (NaDodSO4/PAGE), and the phosphoproteins were detected by autoradiography. Changes in the extent of phosphorylation for each phosphoprotein were determined indirectly by densitometric analyses. The analogous parotid and submandibular 26-kDa membrane phosphoproteins had a rapid phosphate turnover rate (t 1/2 = 5-6 min) whereas the analogous 21-kDa membrane phosphoproteins had a much slower phosphate turnover rate (t 1/2 greater than 20 min). The results of Triton X-114 extraction indicated that the 26- and 21-kDa phosphoproteins were integral membrane proteins. The rate of phosphate turnover for the analogous 26-kDa phosphoproteins is compatible with a regulatory role in exocytosis, whereas the slower phosphate turnover rate for the analogous 21-kDa phosphoproteins suggests that these proteins may play a more subordinate role in secretion or they may coordinate secretion with other cellular metabolic events.

beta-Adrenergic activation of glycosyltransferases in the dolichylmonophosphate-linked pathway of protein N-glycosylation

beta-Adrenoreceptor stimulation of rat parotid acinar cells increases the activity of several microsomal membrane associated, dolichylmonophosphate (Dol-P) linked glycosyltransferases. The activities of Man-P-Dol synthase and Glc-P-Dol synthase are increased by approximately 50%, and the activity of N-acetylglucosaminyl 1-phosphate transferase plus N-acetylglucosaminyl transferase increased by approximately 60%, after agonist treatment. Increases in enzyme activity are (i) independent of endogenous Dol-P levels and (ii) observed under conditions in which the specific activities of donor sugar nucleotides are kept constant. Activation of these enzymes is specific since comparable levels of NADPH-cytochrome c reductase are found in control and agonist-treated membranes. The data thus provide the initial demonstration of neurotransmitter modulation of enzymes in the dolichol-linked pathway of protein N-glycosylation.

Regulation of protein kinases in exocrine secretory cells during agonist-induced exocytosis

Stimulation of exocytosis in exocrine glands is associated with an increased phosphorylation of several particulate proteins. Irrespective of the type of secretagogue (cAMP-dependent agonists, calcium-dependent agonists, calcium ionophores, phorbol esters) exocytosis is always accompanied by an enhanced phosphorylation of the ribosomal protein S6. It is shown by an analysis of the phosphopeptide pattern of the in vivo and the in vitro phosphorylated S6 protein that the protein kinase responsible for phosphorylation of the S6 protein during enhanced exocytosis is protein kinase C. This is so irrespective of whether the agonist uses cAMP or calcium as second messenger. Experiments with isolated guinea pig parotid gland lobules reveal that not only the acetylcholine analog carbamoylcholine, but also the beta-agonist isoproterenol lead within seconds to an increased formation of diacylglycerol. As diacylglycerol increases the affinity of protein kinase C for calcium this finding would explain why the phosphorylation pattern of the S6 protein reflects activation of protein kinase C also under conditions where (as in the case of stimulation with beta-agonists) cAMP is the primary second messenger. It would further explain why the changes of the phosphorylation of individual histones observed during agonist-induced exocytosis in the parotid gland are quite similar for isoproterenol on one hand and carbamoylcholine on the other. A 22 K protein which becomes phosphorylated only when cAMP serves as second messenger is located in the membrane of the endoplasmic reticulum. A possible relationship of this protein with the calcium transport ATPase of the endoplasmic reticulum is under investigation.

Acetylcholine-like effects of 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine ('platelet-activating factor') and its analogues in exocrine secretory glands

The effect of 1-O-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine (1), 1-O-octadecyl-2-acetyl-sn-glycero-3-phosphocholine (2) 1-O-hexadecyl-sn-glycero-3-phosphocholine (3), 1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphocholine (4) and its enantiomer 3-O-octadecyl-2-O-methyl-sn-glycero-1-phosphocholine (5) on the secretion of amylase from guinea pig isolated parotid gland and exocrine pancreatic lobules was examined. Compounds 1, 2 and 4 led to a significant stimulation of amylase release in both systems, effects being already visible between 10-100 pM. Maximal stimulation with compounds 1 and 2 occurred at 5 nM, with compound 4 at 1 nM. Higher concentrations were less effective and at 0.1 microM stimulation was very low. In contrast, compound 5 showed a continuous increase in activity up to 0.01-0.1 microM without a decrease at at higher concentrations. Compound 3 had no effect. For compound 1, its effects on calcium and lipid metabolism have been analyzed and compared with those of the acetylcholine analogue carbamoylcholine. Compound 1 mimicked in every respect the effects of carbamoylcholine. It stimulated the uptake of 45Ca by isolated parotid gland lobules in a non-ionophoretic way. In isolated pancreatic lobules it enhanced the incorporation of [32P]phosphate into phosphatidic acid, phosphatidylinositol and poly(phosphoinositide), increased the formation of diacylglycerols and triacylglycerol, led to the same two-phasic responses of myo-[3H]inositol-labeled polyphosphoinositides, and initiated a rapid short-lasting formation of free inositol triphosphate. Accordingly, 'platelet activating factor(s)' can affect the function of exocrine glands at low concentrations. The effects observed resemble those produced by acetylcholine and result most likely from the interaction of platelet-activating factor with plasma membrane receptors.

Subcellular location of stimulus-affected endogenous phosphoproteins in the rat parotid gland

Rat parotid minces were labeled with [32P]Pi, stimulated with isoproterenol, homogenized in sucrose, and fractionated on continuous sucrose density gradients. We analyzed the resulting fractions by sodium dodecyl sulfate polyacrylamide gel electrophoresis and autoradiograms were made from the gels. Comparison of fractions from control and isoproterenol-stimulated minces revealed seven phosphoproteins that were affected by isoproterenol. The subcellular location of these proteins was determined by comparing their distribution in the sucrose gradients with that of a number of enzymes that are characteristic of specific organelles. Isoproterenol decreased the phosphorylation of two cytoplasmic proteins (Mr 16,000 and 18,000) and increased the phosphorylation of a third (Mr 14,000). The phosphorylation of two endoplasmic reticulum proteins was increased by isoproterenol (Mr 20,500 and 22,500), as was an Mr 31,000 protein which was probably the S6 ribosomal protein. The phosphorylation of a secretory granule protein (Mr 24,000) was decreased by isoproterenol. We then developed a purification scheme for parotid secretory granules. By using this method, we demonstrated that the phosphorylation of the Mr 24,000 was also decreased by carbamylcholine. Granules purified by this method also contained a small number of other phosphoproteins whose phosphorylation was increased only by isoproterenol. Secretory granule-associated stimulus-affected phosphoproteins were found in the particulate fraction when the granules were hypotonically lysed, and were not extracted from the particulate fraction by washing with 0.6 M KCl.

Cyclic AMP mediates beta-adrenergic-induced increases in N-linked protein glycosylation in rat parotid acinar cells

beta-Adrenergic stimulation of rat parotid cells by isoprenaline (isoproterenol) results in 2-3-fold increases in [3H]mannose incorporation into N-linked oligosaccharides. This occurs without perceptible lag and is linear with time for 60 min after agonist addition. Concomitantly, isoprenaline markedly increases cellular cyclic AMP. Examination of individual proteins by sodium dodecyl sulphate/polyacrylamide-gradient-gel electrophoresis reveals that glycosylation changes are primarily associated with four secretory proteins, of approx. Mr 17000, 32000, 38000 and 220000. Beta-Adrenoreceptor activation additionally elicits a slight increase in parotid protein synthesis. The greatest increase in [14C]leucine incorporation is that into another secretory protein (Mr approx. 24000). Exposure of cells to dibutyryl cyclic AMP yields results comparable with those after isoprenaline treatment. Forskolin, which increases parotid-cell cyclic AMP, also causes similar effects. Conversely, dibutyryl cyclic GMP shows no such response. The data are consistent with the notion that beta-adrenergic stimulation of N-linked protein glycosylation in rat parotid cells is mediated by cyclic AMP.

Specific phosphorylation of a protein in calcium accumulating endoplasmic reticulum from rat parotid glands following stimulation by agonists involving cAMP as second messenger

Stimulation of secretion in exocrine cells by agonists involving cAMP as second messenger is associated with the phosphorylation of a specific membrane-associated 22.4-kDa protein (protein III) (Jahn et al.). Here it is shown by subcellular fractionation of rat parotid gland lobules that protein III is associated with the endoplasmic reticulum. The submicrosomal fractions containing protein III, also contain the ATP-dependent microsomal calcium pump activity. Protein III in microsomal subfractions can be phosphorylated in vitro with catalytic subunit from cAMP-dependent protein kinase. Phosphorylated protein III contains exclusively P-serine. Protein III can be removed from ER-membranes with acid chloroform-methanol or Triton X-114, but not by high salt wash indicating that it is tightly associated with the membranes. Protein III is smaller than phospholamban and, in contrast to phospholamban, resistant to heating in SDS. A relationship between phosphorylation of protein III and microsomal calcium sequestration is discussed.

Mode of cytotoxic action of pseudomonal leukocidin on phosphatidylinositol metabolism and activation of lysosomal enzyme in rabbit leukocytes

The cytotoxic action of leukocidin from Pseudomonas aeruginosa was supported by the following observations. (i) The destruction of rabbit leukocytes by the toxin was reduced in the absence of Ca2+ and stimulated by the addition of calcium ionophore A23187 but inhibited by EDTA, EGTA, and TMB-8, an antagonist of intracellular Ca2+ transport. (ii) Uptake of 45Ca into leukocytes exposed to the toxin was enhanced about threefold the rate of uptake into untreated cells. The increased 45Ca uptake into the cells was slightly inhibited by trifluoperazine, an inhibitor of Ca2+-calmodulin activity, but not by ruthenium red. (iii) Pseudomonal leukocidin enhanced rapidly the labeling of phosphatidylinositol, polyphosphoinositides, phosphatidic acid, and lysophosphatidic acid from [32P]phosphate. The time course experiments of the labeling and breakdown of these phospholipids suggested that the initial action of this toxin was to stimulate phosphatidic acid production, presumably causing a rapid metabolic change of phosphatidylinositol correlating with the activities of phosphatidylinositol-specific phospholipase C and 1,2-diacylglycerol kinase. It was considered that a rapid formation of phosphatidic acid and degradation of polyphosphoinositides might be related to a Ca2+ movement from extra- and intracellular space. (iv) In leukocytes exposed to the toxin, acid phosphatase activity as a marker enzyme of lysosome was activated up to 75% of the lysosomal enzyme before cell destruction. The leakage of lysosomal enzyme from the cells occurred at the almost same time as leukocyte destruction. The mode of cytotoxic action of pseudomonal leukocidin is discussed.